How to Measure Choke Coil Power Factor in the Lab

There are two common laboratory methods to measure the power factor of a choke coil.

Method 1: Voltmeter–Ammeter–Wattmeter (VAW) Method

This is the most direct method.

1. Circuit: Connect an AC supply in series with an ammeter, the choke coil, and the current (series) coil of a wattmeter. Connect a voltmeter and the wattmeter’s pressure (voltage) coil in parallel across the choke coil.
2. Measurements: Apply voltage and record:
   • Voltmeter reading = V (volts)
   • Ammeter reading = I (amperes)
   • Wattmeter reading = P (watts)
3. Calculations:
   • Apparent power S = V × I
   • Real power P = wattmeter reading
   • Power factor (cos φ) = P / S = P / (V×I)

Method 2: Three-Voltmeter Method

This method is useful if a wattmeter is not available.

1. Circuit: Connect the choke coil in series with a known, non-inductive (pure) resistor (R).
2. Measurements: Using one voltmeter, take:
   • V_R (across the known resistor)
   • V_C (across the choke coil)
   • V_T (across the total series combination)

3. Calculations:

   From the phasor (voltage) triangle and the Law of Cosines: 
   V_T² = V_R² + V_C² + 2V_RV_Ccos φ

   Hence the choke coil power factor is: 
   cos φ = ( V_T² + V_R² - V_C² ) / ( 2 V_T V_R )

   Here, φ is the phase angle between the choke coil voltage and the current; for a series test with a non-inductive resistor, this equals the coil’s power-factor angle.